In general, in image-forming apparatuses such as a copying machine and a laser beam printer using electrophotography, an image-forming process is performed in which a toner image is formed on a transfer-receiving material (such as recording paper) through an exposure step, a developing step, and a transfer step, and the toner image is then fixed. In a widely used fixing method, a transfer-receiving material having an unfixed toner image thereon is caused to pass between a fixing roller and a pressure roller, is pressed between the two rollers, and is heated by a heating source provided in the fixing roller.
As the fixing roller, for example, PTL 1 discloses a steel use stainless (SUS) tube. However, such an SUS tube has problems in terms of cost because the material cost and the processing cost thereof are high. Furthermore, an SUS tube has poor flexibility and has a problem in that, for example, cracks are easily generated at an end or another portion of the tube when paper jamming or the like occurs. To address these problems, a tube composed of a heat-resistant resin, in particular, a polyimide tube including a base composed of a polyimide resin, which has good heat resistance, mechanical strength, etc., has been proposed. An endless belt (fixing belt) including a polyimide tube having flexibility and a fluororesin layer for providing a releasing property, the fluororesin layer being disposed on the external surface of the polyimide tube, has been widely used.
FIG. 1 is s cross-sectional view that schematically illustrates a fixing step using such a fixing belt. As illustrated in the figure, an unfixed toner image 5 on a transfer-receiving material 4 is heated by heating means (a heater) 2 through a fixing belt 1, and is fixed by applying a pressure with a pressure roller 3 to form a fixed toner image 6.
In order to reduce the waiting time after a power supply is turned on, to reduce the electric power consumption, to realize a high fixing speed, to realize a low fixing temperature, etc., high thermal conductivity is desired for the fixing roller. To improve the thermal conductivity, blending a thermally conductive inorganic filler in a polyimide tube has been proposed.
For example, PTL 2 discloses a fixing belt including an inner layer which is a tube composed of a polyimide containing an inorganic filler having high thermal conductivity, and a fluororesin layer disposed on the external surface of the inner layer. Boron nitride (BN) and other compounds are described as specific examples of the inorganic filler.
Regarding a polyimide tube used for a fixing roller, PTL 3 also discloses an example in which a polyimide containing BN and a needle-like, inorganic, thermally conductive filler is used, and PTL 4 discloses an example of a tube composed of a polyimide containing a needle-like, inorganic, thermally conductive filler, in which the needle-like, inorganic, thermally conductive filler is oriented in the circumferential direction of the tube. Furthermore, PTL 5 discloses a polyimide tube in which a needle-like, inorganic, thermally conductive filler oriented in the circumferential direction is dispersed in an amount of 15% by volume or more, and a method for producing the polyimide tube, in which a dispenser method is used.